The fast thermal stopped-flow appparatus has been rebuilt to eliminate the temperature rise produced upon stopping. By coupling the drive syringes to two identical output syringes a zero pressure gradient across the observation tube is produced upon stopping. The total temperature artifact due to flow and stopping is now less than 1 millidegree C. Utilizing FEST (Finite Element Simulation Technique) sensor response of 7 milliseconds to 1/e has been corrected to 3.5 milliseconds and is limited mainly by the 3 to 4 millisecond dead time of the instrument.